1. Technical Field
The present invention relates to a power supply apparatus having a battery pack with a plurality of rechargeable batteries connected in series, and a charge circuit which charges the battery pack.
2. Description of Related Art
FIG. 1 shows a circuit diagram of a battery pack 21 having a plurality of batteries 23 connected in series. The battery pack 21 has an overdischarge protection circuit 26 which prevents overdischarge of each battery 23. The overdischarge protection circuit 26 detects the voltage of each battery 23. When the voltage of any battery 23 becomes lower than a minimum voltage, the overdischarge protection circuit 26 turns a discharge current interruption FET 24 to OFF state. The discharge current interruption FET 24 turned to OFF state cuts off the discharge current, stopping discharge. The battery pack 21 is connected to a charger 22, and is charged. A charge current passes through a intrinsic diode 25, (in other ways, the diode 25 is called a parasitic diode 25) of the discharge current interruption FET 24, and flows into the batteries 23.
Since the discharge current interruption FET 24 is an equivalent circuit in which the parasitic diode 25 is connected in the reverse direction in parallel, it can cut off a current in the forward direction. However, the discharge interruption FET 24 cannot cut off a current in the reverse direction. Accordingly, the discharge current interruption FET 24 can pass the charge current in OFF state, where discharge is prohibited, so the battery pack 21 can be charged.
However, the parasitic diode has the disadvantage of large electric power loss as compared with the case where a current flows through the FET in ON state in the forward direction. A relatively large voltage drop occurs in the parasitic diode in the case where a current flows in the forward direction. Additionally, electric power loss in the parasitic diode increases in proportion to the product of a voltage drop and a current. Since the voltage drop in the parasitic diode is large as compared with the discharge current interruption FET in ON state, electric power loss in the parasitic diode is also large. Accordingly, in the case where the battery pack is charged by passing the charge current through the FET in OFF state via the parasitic diode, the amount of heat generation of the FET is very large. As a result, adverse effects arise, such as a thermal breakdown or deterioration of the discharge current interruption FET due to the charge current when the battery pack is charged.
A charger, which detects the output voltage of a battery pack before charging it and controls the charge current with the amplitude of output voltage, has been developed. This charger charges the battery pack whose output voltage is lower than a prescribed voltage with a small current in a preliminary charge mode, not in a quick charge mode. The charger switches to the quick charge mode when its output voltage becomes higher than the prescribed voltage due to preliminary charging and charges the battery pack with a large current. The charger does not charge a battery pack whose output voltage is low and which may be in trouble with a large current in the quick charge mode. Thus, the charger can safely charge a battery pack.
However, as shown in FIG. 1, in the case where this type of charger charges the battery pack 21 which switches ON/OFF of the discharge current interruption FET 24 based on the voltage of each battery. The charger may start charging the battery pack 21 in the quick charge mode in the state where the discharge current interruption FET 24 is OFF state. Generally, the voltage is set so that the charger switches to the quick charge mode after the FET turns to ON state sufficiently. But, when cell balance is lost, problems arise. For example, a battery pack is charged in the quick charge mode from the start when the battery pack, whose output voltage is higher than a prescribed voltage to start quick charging though the voltage of any battery, becomes lower than a minimum voltage, and the discharge current interruption FET is in OFF state. In the case of the battery pack in which the voltage of any battery become lower than the minimum voltage, a possibility that the output voltage becomes lower than the prescribed voltage increases. However, as the number of the rechargeable batteries connected in series increases, the voltage of one battery less affects the output voltage of the battery pack. For this reason, even if the voltage of any one battery becomes lower than the minimum voltage, the output voltage of the whole battery pack may be higher than the prescribed voltage to start quick charging. When the battery pack in this state is charged in the quick charge mode, the charge current flows through the parasitic diode in the discharge current interruption FET in OFF state. When a large current for quick charging flows through the parasitic diode, the amount of heat generated in the discharge current interruption FET should be large, which causes thermal runaway and breakdown.
In order to solve the deficiencies noted above, a device has been developed that turns a discharge current interruption FET in OFF state to an ON state for quick charging.
(e.g., Japanese Patent Laid-Open Publication TOKUKAI No. 2001-57743, hereafter “Tokukai”)
When charging the battery pack in which the discharge current interruption FET is in OFF state, the device disclosed in Tokukai turns the discharge current interruption FET from OFF state to ON state and charges it in the quick charge mode. Current is applied to the battery pack charged in the quick charge mode in this state and not through the parasitic diode. Thus, heat generation by the parasitic diode can be eliminated. The reason is that, the discharge current interruption FET in ON state is connected to the parasitic diode in parallel. Thus, the voltage drop is very small as compared with the parasitic diode. This can reduce power consumption.
However, this device forcedly turns the discharge current interruption FET in OFF state to ON state, and charges the battery pack with a large current in the quick charge mode. Accordingly, although the device can prevent heat generation of FET, it cannot safely charge all of battery packs. The reason is that a battery pack in which the voltage of any battery becomes lower than the minimum voltage is charged with a large current in the quick charge mode. The battery whose battery voltage is lower than the minimum voltage is an abnormal battery or a battery which deeply discharges. In the case where the battery is not an abnormal battery but a battery which deeply discharges, when the battery is charged with a small current in the preliminary charge mode, its battery voltage becomes higher than the minimum voltage. In the case of an abnormal battery even if the battery is charged in the preliminary charge mode, its battery voltage does not rise high from the minimum voltage. Accordingly, it is preferable to charge a battery pack in which the voltage of any battery becomes lower than the minimum voltage in the quick charge mode after it is confirmed that the voltages of all batteries become higher than the minimum voltage. In other words, after it is confirmed that the battery pack is not abnormal. In this case, it is preferable to initially charge the battery pack with a small current in the preliminary charge mode. On the contrary, a device, that forcedly turns the discharge current interruption FET from OFF state to ON state to charge a battery pack in the quick charge mode, charges even a battery pack containing an abnormal battery, which cannot be safely charged.